High-current precision current sources are useful for such applications as light emitting diode (LED) drivers. This is because LEDs tend to draw relatively high levels of current, e.g. in the 1+ amperes range, and operate best with a precise, constant and steady current supply.
FIG. 1 is a block diagram of a prior art integrated circuit (IC) 10 including low-side current control circuitry 12 for an LED 14. IC 10 also includes a PMOS transistor 16 and a sensing resistor (RS) 18 which couples a cathode of LED 14 to ground. In operation, the control circuitry 12 senses the voltage drop across sensing resistor 18 and provides a control signal on the gate of PMOS transistor 16 to regulate the flow of current through the LED 14.
FIG. 2 is a block diagram of a prior art integrated circuit 20 including low-side current control circuitry 22 and an NMOS transistor 24. The control circuitry 22 senses the voltage across a sensing resistor (RS) 26. An LED 28 is coupled between the sensing resistor 26 and the drain of NMOS transistor 24. The control circuitry 12 regulates the flow of current through LED 24 by applying a control signal to the gate of NMOS transistor 24.
A disadvantage of the circuits of FIGS. 1 and 2 is that the high currents that drive the LEDS flow through the sensing resistors RS. Since the power consumed by a resistor is I2R it is therefore desirable to make the sensing resistors RS as low in resistance as possible, e.g. less than about 1Ω. Because of the low resistance of the sensing resistors they typically are precision resistors which are expensive and hard to provide on an integrated circuit. Furthermore, even though their resistances are very low, high currents flowing through sensing resistors RS result in a significant voltage drop. For example, if 1 ampere is flowing through a 1 ohm sensing resistor, the voltage drop across the resistor will be 1 volt. This voltage drop across the sensing resistor, referred to herein as “drop-out”, reduces the potential voltage swing across the LEDs being driven by the circuits.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.